This application is related to and claims priority from Japanese Patent Applications No. Hei. 11-166962 filed on Jun. 14 1999, and No. Hei. 11-321479 filed on Nov. 11, 1999, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an actuator having a slipping prevention unit which prevents a slipping of an output shaft due to an external force from a load side. The actuator is suitably used for a vehicle brake device, for example.
2. Description of Related Art
In a conventional brake device operated by an actuator using an electrical motor, an electrically driven parking brake mechanism is provided. For example, in a brake device described in U.S. Pat. No. 5,219,049, when a parking brake is braked, a slipping of an output shaft of an actuator is prevented while a brake is operated by the actuator. However, the structure of a parking brake mechanism is complicated, and many components including both friction plates are necessary for the parking brake mechanism.
In view of the foregoing problems, it is an object of the present invention to provide an actuator having a slipping prevention unit which is constructed with a simple structure and prevents a slipping of an output shaft due to an external force from a load side.
According to the present invention, actuator having an output shaft for driving a load includes a driving motor for driving and operating the output shaft, and a slipping prevention unit which prevents a slipping of the output shaft due to an external force from a load side. The slipping prevention unit includes a rotation regulation unit having an engagement portion engaging with a rotation member, and a switching unit which electrically switches operation of the rotation regulation unit between an engagement state where the engagement portion of the rotation regulation unit engages with the rotation member, and a non-engagement state where the engagement portion is disengaged with the rotation member. The engagement portion has a contact surface in a rotation direction for contacting the rotation member, and the rotation regulation unit is disposed so that at least one side rotation of the rotation member is prevented in the engagement state. Thus, when the engagement portion engages with the rotation member by the switching unit, at least the one side rotation of the rotation member becomes impossible. As a result, the actuator prevents a slipping of the output shaft due to the load side, with a simple structure of the slipping prevention unit.
Preferably, in the engagement state, an end surface of the engagement portion in the rotation direction contacts a regulation wall of a housing for accommodating the slipping prevention unit. Therefore, in the engagement state, a rotation of the rotation regulation unit is accurately prevented, and the rotation of the rotation member is accurately prevented.